The objective of the proposed research is to secure definitive information at the molecular level about the modulation of activity of the plasma protease inhibitor, antithrombin III. Specifically, it is proposed to test the hypothesis that a post-translational modification of antithrombin III is a necessary prerequisite to its becoming a functionally active inhibitor of the activated coagluation factors and to investigate in more detail the molecular nature of the thrombin-antithrombin III complex. First, an extensive study will be made of the ability of antithrombin III to refold into its active conformation after denaturation by urea or by temperature. These experiments will determine if there is sufficient information in the amino acid sequence of antithrombin III as isolated from plasma to re-form the active inhibitor. Second, a search shall be initiated for a latent form of antithrombin III in plasma by utilizing immunoaffinity chromatography. Third, physical-chemical studies shall be performed on purified thrombin-antithrombin complex to illuminate the molecular changes which occur in each protein upon complex formation and to learn more about the nature of the chemical bonds which maintain the integrity of the inactive enzyme-inhibitor complex. It is anticipated that the information gleaned from this research will contribute toward the understanding of the pathogenesis and management of thrombo-embolic disease.